The present invention relates to ground based pulse radar systems and methods, and more particularly to the operation of airport-traffic-control radar systems which provide better moving-target-indications with better rejection of ground, meteorological, and other clutter echo signals.
In radar detection of aircraft or other moving targets, a pulse radar system functions to measure the position and range of a detected target on the basis of the time it takes for a pulse to reach the target and reflect back to the radar system. Theoretically, the operative target detecting range of a radar system depends primarily on the radar transmitter power, the antenna gain (or narrowness of beam width), the radar cross section of the target, and the received echo power (or the receiving antenna aperture and the receiver signal sensitivity).
However, other parameters enter into determining the actual range of a practical radar system. Transmitted pulse repetition frequency (PRF), set in the radar system design, normally limits the range of a radar system. Thus, the radar range may be limited by range ambiguities which may arise where the PRF is so high that echo signals from one pulse may return after the next pulse has been transmitted.
From the standpoint of the radar environment, clutter objects in the illumination field of the radar system produce clutter echo signals which significantly limit the effective moving target range of the radar system depending on the clutter rejection capability of the radar system. Thus, various ground and other objects return echo signals indicative of stationary clutter, moving objects such as rain or other meteorological objects return echo signals indicative of moving clutter, and moving targets return echo signals which are to be processed by the radar system receiver for display after separation from the clutter return signals.
The moving target echo signals accordingly may be detected and discriminated from extraneous signals, i.e., stationary and moving clutter echo signals and interference signals. Conventional radar systems have moving target detection capabilities which are limited in target detection range, clutter discrimination and rejection, and detection of otherwise falsely rejected moving targets.
The total radar cross section of clutter signals typically increases with distance from the radar transmitter. In the current state of the airport traffic control (ATC) or airport surveillance radar (ASR) art, the clutter cross section may be as great as one thousand or more times the radar cross section of a moving aircraft within the field of illumination and within the actual range of an actual radar system.
The effective moving target detection range of current, S-Band ATC radar systems may, at most, be about 50 to 60 miles on the basis of current moving aircraft detection and clutter rejection capabilities.
The speed of light is the basic factor which places this range limit on state-of-the-art ATC radar systems, with use of a PRF of 18 to 24 pulses per beam width of 1 to 2 degrees at an antenna scan rate of 60 degrees per second. To achieve significantly greater moving target detection range for airport traffic control or surveillance, military air surveillance, and other similar radar applications, a radar system must operate with significantly better clutter discrimination capability at a lower PRF. Other classes of radar systems, such as a class known as the TPJ 70, similarly cannot handle moving clutter and have a need for better clutter discrimination capability.
More particularly, ATC radar systems have typically been embodied as the moving-target-indication (MTI) type or the more recent moving-target-detector (MTD) type to monitor and control aircraft takeoffs and landings at airports. In these systems, the doppler shift in the echo pulse frequency of a moving aircraft target is used to distinguish the moving target from stationary objects or clutter even though the echo signals from the clutter may be much greater than the echo signals from the moving target.
The radar system circuitry extracts doppler information from the transmitted and target echo signals, and determines moving target position and velocity from the doppler information.
The MTI radar system is a pulse radar system which mixes a coherent (COHO) reference signal with down converted echo signals returning from ground or other stationary clutter, moving clutter, and moving targets and any received noise and interference signals. An output signal from the mixer represents doppler information, since a mixer output signal includes a DC signal, representative of stationary clutter, and a varying signal, representative of moving targets and moving clutter. The varying signal occurs to the extent that the frequency of the echo signals differ from the COHO reference frequency (i.e., the original pulse transmission frequency) as a result of motion of objects from which the echo signal is received.
In addition, the mixer aliases moving target echo signals as zero doppler signals when the target is moving at a speed which results in a position change equal to an integral number of half-wavelengths of the transmitting frequency. In other words, conventional MTI radar systems falsely reject moving target echo signals when the target has a stroboscopic appearance of being still and when the radar system cannot independently detect a position change because the limited amount of target movement between transmitted radar pulses is less than the position resolution of the radar system.
Known MTI radar systems therefore use additional processing in an attempt to detect all moving target echo signals while separating moving target echo signals from clutter, and noise signals for output display. MTI radar systems often employ variable timing between transmitted pulses on a pulse by pulse basis to circumvent false rejection of moving target echo signals. However, this approach has limited effectiveness because it is coupled with filtering fixed at the time of design as described below.
Further, conventional MTI radar systems do not address rejection of moving clutter and thus cannot reject moving clutter doppler signals while passing moving target doppler signals. The conventional MTI system output display thus contains both moving target and moving clutter images, and the system operator must distinguish each from the other.
Prior-art MTI radar systems use hardware or software high-pass filtering to block clutter based on its expected spectral characteristics. The filter characteristics are normally fixed in the design stage of a conventional MTI radar system to reject optimally a specific type of clutter or a specific group of clutter types. A CFAR (constant false alarm receiver) may be provided to allow the system target detection threshold to ride above the noise-plus-clutter residue level thereby characterizing the MTI radar with limited adaptivity at the CFAR level but not at the filtering level.
In any case, if the actual clutter spectrum differs from the spectrum assumed for MTI radar design purposes, system degradation occurs during MTI radar system use for moving target detection. Such degradation may result from failure to remove clutter outside of zero notches, or from excessive attenuation of moving target signals due to an excessively wide clutter notch, or from a combination of these two effects.
Overall, conventional MTI radar systems have had limited effectiveness in rejecting clutter and in detecting and displaying moving targets for ATC and other applications. Further, the effective moving target detection range has been significantly limited by system design compromises needed to reach such limited target detection effectiveness. Although MTI radar systems employ variable interpulse spacing to provide some flexibility in the illumination waveform, these systems provide no filter adaptivity to actual clutter conditions as explained above.
The conventional MTD radar system is another kind of system which has been used and is currently being widely used for moving target detection in ATC and other applications. In MTD radar systems, pulses are transmitted at a fixed frequency in successive pulse bursts, and the pulse frequency can be varied from burst to burst.
Conventional MTD radar systems employ filter banks to achieve clutter rejection with some increase in adaptivity, as compared to conventional MTI radar systems. Thus, independent CFAR detectors can be used on the outputs of the respective filters, and the filter response can be varied with limited adaptivity toward matching actual clutter characteristics.
However, the actual filter characteristic shapes must be specified in the MTD system design stage to optimize removal of expected clutter spectra as opposed to actual clutter spectra to be faced in system operation. In this connection, parameters such as sidelobe level and main lobe width and position must also be set in the design stage.
Although the prior-art MTD radar systems have some additional adaptivity beyond that available in conventional MTI systems, such MTD systems have had limited effectiveness in detecting moving targets due to limited moving target detection range and restrictions imposed by batch PRF processing. For example, each pulse burst has a fixed PRF with fixed interpulse time periods in each pulse burst, and the PRF can only be changed in the time between pulse bursts. This restriction significantly limits the capability of the MTD radar system to find falsely rejected moving targets which are moving at a blind speed(s) for which echo signals are blocked by the filter bank. In other words, MTD radar systems have lacked flexibility in finding satisfactory illumination waveforms which enable good target detection coupled with good clutter rejection under actual operating conditions.
In addition, filter banks like those used in conventional MTD radar systems are suitable for use only with a constant PRF. Thus, clutter rejection regions formed by the filter bank are replicated at multiples of the PRF. Several bursts of radar illumination pulses, each burst with a different PRF or carrier frequency, are required for each radar search resolution cell to provide detection coverage for blind speeds. Since the pulse bursts must be combined noncoherently, additional observation time, implied by the multiple bursts, fails to provide improved spectral or cross-range resolution.
In view of the state of the ground based, pulse radar system art, and specifically the ATC radar system art, a need clearly exists for an improved ground based, pulse radar system characterized with better moving target detection, extended moving target detection range, and better clutter rejection based on better adaptivity to actual clutter conditions. This need especially applies to airport traffic control since meeting it translates into better airport controller operations and better aircraft safety based on reliable detection of all aircraft at greater distances regardless of clutter and weather conditions.